There exist many examples of flow measurement through a fixed channel or tube. The simplest of these techniques utilizes a time of flight measurement, thereby only measuring flow through a channel for a fixed time period. Such sampling, therefore, results in finite flow measurements, incapable of accounting for dynamic variations in fluid flow during those periods outside of the time of flight measurement period.
More advanced techniques utilize an external excitation and measurement means. In exciting a fluid, a heating means is typically utilized, wherein such a heater is capable of delivering thermal energy to a closed fluid passageway. Two or more thermocouple probes, located upstream and downstream of the heating source, are then use to measure the local temperature differences at fixed points along the flow channel. Utilizing these components, a temperature gradient for a stagnant and flowing fluid can be obtained. Based upon this data, a finalized fluid flow may be calculated.
While the localized heating and temperature measurement of a fluid, as set forth above, is useful in a variety of industrial applications, the overall flow measurement accuracy is limited by several system inherent sources of error. Firstly, under the aforementioned flow measurement technique, system accuracy is directly related to thermocouple accuracy. To accurately determine fluid flow, each thermocouple must be able to discriminate between discrete differences in fluid temperatures at the associated thermocouple position. In systems in which there exists a large heat introduction in a quickly moving fluid, such temperature differences are greatly apparent. In light of this, determining fluid flow to the required degree of accuracy is made simple. In a setting in which there exists a small fluid flow velocity, the upstream and downstream temperature variations are greatly reduced. One such setting may be seen in a High Performance Capillary Liquid Chromatographic (HPLC) setting, in which fluid flows of less than 1 μL/min are not uncommon. In light of such a decrease in temperature variation, it becomes important to measure temperature to a much higher degree of accuracy. Such an increase in accuracy requires the use of a significantly more expensive thermocouple device.
Furthermore, under a temperature based system, there exist additional sources of system error which are not easily prevented. One such source of error lies in the thermal conduction of heat through the walls of the fluid channel. Such conductive heat transfer from heat source to thermocouple position results in the loss of flow measurement accuracy, as the thermocouple is no longer solely recording fluid temperature, but rather is under the influence of additional heat addition through the walls of the flow channel. Convective losses of heat along the flow channel exterior additionally contribute to the inherent system inaccuracies. Should a user elect to use externally mounted thermocouples, which are seated along the external surface of the flow channel, additional error is introduced as said external thermocouples are merely recording the fluid boundary layer temperature, as opposed to the interior fluid temperature at points away from the boundary layer. Numerous attempts have been made in the art to prevent these conductive and convective losses, all of which result in increases in system cost and complexity.
In light of the above, when operating in a low flow environment utilizing thermocouples as sensor means, the use of an internal thermocouple element is important in providing the greatest degree of flow rate measurement accuracy. Such internal thermocouples are in direct physical contact with the flowing fluid and offer great sensitivity and time response. Direct contact between an internal temperature probe and fluid, however, results in potential contamination of the fluid or of the temperature probe element. As the fluid is in direct contact with the internal probe, particulate matter suspended in a flowing fluid may contaminate the external temperature sensing region of a temperature probe, thereby resulting in inaccurate measurements. Furthermore, in the presence of highly corrosive or chemically reactive fluids, contact between the internal temperature probe and these fluids may result in the break down of the internal temperature probe surface, thereby resulting in the introduction of contaminant into the flowing fluid.
Additionally, the use of direct contact thermocouples require the introduction of numerous fittings, couplings and related alterations to the flow channel to adequately introduce the temperature measuring probes to the fluid flow. These extraneous additions introduce numerous sources of failure or leakage. In a high pressure operating environment, such as a HPLC setting, the addition of flanged or threaded couplings to a fluid channel requires skilled assembly of precision components. Such fittings introduce several potential failure locales when compared to a continuous, uninterrupted flow channel. Additionally, the introduction of these aforementioned temperature sensors into the fluid channel greatly increases system costs and complexity.
Finally, in a liquid chromatography environment, the addition of these aforementioned couplings, fittings and invasive temperature elements to the fluid pathway greatly increase the “dead volume” of the chromatographic column. The term “dead volume” is used to describe the unknown volume which is trapped in these various fittings, couplings, and thermocouple regions of the flow channel. This fluid may be either stagnant or dynamic in nature and may unpredictable escape into the fluid channel thereby causing the shape of the fluid pulse to be altered from the desired shape.